There are numerous prior art arrangements in which a common data bus is used to establish signal communication between separate systems, system components or subsystems. In such arrangements, each system, subsystem or system component ("utilization device") is connected to the common data bus thus allowing the utilization devices to communicate with one another and/or share common resources such as a memory or a signal source without separately connecting the utilization devices to one another or separately connecting the utilization devices to a commonly employed signal source or memory. Thus, the use of a common data bus can greatly simplify system topology, often resulting in increased reliability and reduced costs of installation and repair. Since time division multiplexing of the data bus signals can be employed, such arrangements also are capable of providing relatively high speed signal communication between a relatively large number of utilization devices.
The ability to interconnect numerous utilization devices with a common data bus (e.g., a single pair of wires) instead of numerous individual wires is of special significance relative to the avionics and electronics systems of modern aircraft. Specifically, using one or more common data buses to interconnect various aircraft avionics and electronics systems and/or to interconnect the subsystems and components of a single aircraft avionic or electronic system can provide a substantial reduction in the space required for the routing of wires and the weight of the necessary wires. Further, using one or more common data buses allows traditionally separate systems that utilize independently derived, identical signal information to be integrated with one another to the extent that signal information generated by one of the utilization devices can be transmitted to other utilization devices via the common data bus thereby eliminating the need for independent derivation or replication of that signal information. Such integration permits simplification of various utilization devices, with further attendant reduction in aircraft weight and further reduction in the space required for aircraft electronic and avionic systems.
Various common data bus data communications have been proposed for use onboard aircraft. For example, U.S. Pat. Nos. 4,199,663 and 4,471,481, both entitled "Autonomous Terminal Data Communication System" and assigned to the assignee of the present application, disclose data bus communication systems specifically arranged for the integration of various aircraft electronic and avionic systems and the achievement of significant reduction in the amount of wiring required to interconnect the various utilization devices of the data communication system. Moreover, these prior art references disclose a system communication protocol which provides reliable and efficient data communication in an aircraft environment.
U.S. Pat. No. 4,264,827 entitled "Current Mode Data or Power Bus," also assigned to the assignee of the present application, discloses autonomous terminal data communication systems that utilize a current mode data bus and are well suited for airborne application. In the arrangement of U.S. Pat. No. 4,264,827 a pair of twisted wires forms the current mode data bus. Coupling transformers having ferrite cores that can be disassembled so that the two conductors of the twisted pair data bus can be inserted between the legs of the disassembled cores provide the inductive coupling of signals to and from the data bus. Specifically, in this arrangement, the data bus wires form secondary transformer windings with the primary windings being permanently installed on the ferrite cores and being connected to data transmitter and/or receiver circuits of a utilization device. One important result of this arrangement is the establishment of signal coupling without the need to cut the conductors of the current mode data bus or having to remove or perforate insulation of those conductors.
U.S. Pat. Nos. 4,823,364 and 4,825,450, respectively entitled "Receive Coupler for Binary Data Communication System" and "Binary Data Communication System" (each of which is assigned to the assignee of the present application), disclose specific signal modulation techniques and arrangements for coupling signals to and from a data bus that is formed by a pair of twisted wires. In the arrangements disclosed by these two references, signals are coupled to and from a current mode data bus by a transmit coupler and a receive coupler, each of which is inductively coupled to the data bus by means of a transformer. Each transmit coupler includes circuits identified as a stub driver and a line driver, which are connected to one another by a shielded twisted pair of wires. The stub driver receives signals from an associated utilization device and converts or conditions the signals for voltage mode transmission along the shielded twisted wire pair. The line driver amplifies the signals provided by the stub driver (via the shielded twisted wire pair) and applies the amplified signal to the data bus via a transmit coupler transformer. The disclosed receive couplers are of similar topology, including a receive coupler transformer; a bus receive amplifier; a shielded twisted pair of wires and a stub receiver. Signals propagating along the data bus bus are coupled to the bus receive amplifier via the receive coupler transformer. The signals provided by the bus receive amplifier are coupled to the shielded twisted wire pair for voltage mode transmission to the stub receiver. The stub receiver provides the necessary signal conversion or conditioning and supplies the conditioned signals to an associated utilization device.
Although the prior art digital communication systems such as those disclosed in the above-referenced United States patents are suitable for use in aircraft and, have at least partially eliminated previous problems, a need for additional improvement exists. For example, the two shielded pairs of twisted wires included in receive and transmit couplers of the type disclosed in U.S. Pat. Nos. 4,823,364 and 4,825,450 result in several system limitations. First, because of signal attenuation and other factors, the length of the shielded twisted pair utilized in the transmit and receive couplers is limited to approximately 30 meters (about 100 feet). In some airborne data bus applications, it is desirable or necessary to connect a data bus to utilization devices that are located more than 30 meters from the data bus. The significant signal attenuation of the shielded twisted pairs can also impose stringent signal conditioning requirements, both with respect to the receive coupler stub receiver amplifier and with respect to the transmit coupler line driver. Moreover, since the shielded wires exhibit a relatively low characteristic impedance, relatively high signal currents are required. Thus, the power level of the transmit channel stub driver and the receive channel receiver amplifier often is higher than desired and circuit efficiency is less than optimal.
Because prior art aircraft data communication systems of the above-described type employ an individual transmit coupler and/or receive coupler for each system utilization device and because a relatively large number of utilization devices can be located within a confined region of the aircraft, it sometimes is necessary to "cluster" numerous couplers along a relatively short length of the data bus. Such clustering can result in signal reflections that decrease the system signal to noise ratio. Even in situations in which this reflection noise does not substantially degrade system performance, the requirement that each utilization device employ a separate receive and/or transmit coupler can result in system topology that is more complex than desired.
As is known in the art, to minimize system complexity and weight, prior art aircraft data communication systems typically supply operating current to the coupler circuitry via the same wires that carry the transmit and receive signals to and from the data bus. For example, in the data communication system disclosed in U.S. Pat. No. 4,825,450, the operating current for the line driver and receive amplifier, which are located proximate the data bus, is supplied via the conductors of the two individually shielded pairs of wires that carry the transmit and receive signals. In this arrangement, resistors are utilized to decouple the power supplies from the data communication signals. This power supply/decoupling arrangement is at least somewhat undesirable because it results in a relatively low common mode rejection voltage range which can have deleterious effect on system susceptibility to electromagnetic interference. Moreover, any imbalance in the receive and transmit coupler arrangements can result in conversion of common mode electromagnetic interference signals into differential interference signals that are amplified by the receive amplifier and thereby degrade system performance.